A Shallow Kind of Green
By Martine van Rooyen and Johan Wentzel
The English TV show ‘Grand Designs’ should be accompanied by a warning for viewers. Watching this programme will make you feel that you could, and should, also build that slightly different masterpiece that will become a great family home. So when we returned to South Africa, after 10 years of living in the UK, we aimed to do just that.
The ideal was to be as green and self-sufficient as possible, showing that with a few small changes you can live comfortably, but with a much smaller ‘footprint’.
Since we had moved to ‘the bush’ to enjoy nature it felt right to make the house blend in with its surroundings; a green roof would be ideal for this. Our house is running entirely on solar power, so keeping the internal temperature as constant as possible was also important. Because of the required solar panels however, we could not make the whole roof green and opted to only make the larger south-facing part (245m2) green.
Inspired by the GluckPlus ‘house in the mountains’, we aimed to create a sloping roof, which would nearly reach to the ground. Balancing the minimal required height indoors with a wish to keep the house on the outside as low as possible led to a roof incline of 7%. While a sloping roof makes getting rid of excess water somewhat easier, it also means that anti-erosion measures will have to be applied.
Next came the question whether to go for an intensive or extensive green roof. Since we wanted to make the house blend in with the veld on our plot and we are lucky to have many trees on our plot that are approximately equally high as the house, we felt that extensive would be the right option for us. Working on a carrying capacity of 550kg per m2, we ended up with a concrete slab, which is on average 20cm thick, which allows for a soil depth of 20cm. This soil depth allowed us to plant various grasses, perennials and small shrubs, which all occur naturally in the area.
To prevent any leaks the waterproofing contractor applied two layers of Polyglass to the top of the concrete slab and the concrete upstands surrounding the slab. The first layer was Polyglass Evolight onto which Polyglass Antiroot was bonded. He gave us the valuable advice that even a small stone caught in the sole of a shoe could damage the waterproofing and cause a leak, so we minimized traffic onto and off the roof during installation.
Next came the Geotech materials, we were especially focusing on drainage and anti-erosion. For this we used the following products: Dimpledrain, the small grooves in this “carpet” made sure the water could get away quickly, whereas the Bidim on top prevented the soil from clogging up the grooves. At the lowest side of the roof we used Megaflo, a product that acts as a drainpipe, but can be hidden in the soil (this was also wrapped in Bidim). And finally, we added Multicell pockets on top to make sure the soil would stay in place even with heavy rainfall.
To minimize the weight on the roof after heavy rainfall, we chose a very light soil made up of 40% composted bark, 40% compost, 5% topsoil, 15% river sand and organic fertilizer. So far this soil has dealt very well with rainfall. Depending on the intensity of the precipitation, excess water will drip out immediately or up to a day later. The plants are growing vigorously. The only drawback is that a few small bark chips blow off the roof in heavy wind.
And finally, the issue of timing. We were advised to be finished with the planting by end January, to give the plants time to establish before the dry season began. Another question was how to get everything onto the roof and in what order, since you cannot heap too much soil in one place because of the weight; but we were also concerned that unweighted Geotech materials might be blown away overnight. So we divided the roof into several areas and finished one area completely before starting with the next.
Since the “bush” was retained on site wherever possible, it only made sense the use local plants for the roof as well. However, certain restrictions had to be considered. The two main ones were the carrying capacity of the concrete roof and the root system of the plants to be used. This limited the selection the smaller species of the surrounding Veld type, namely Marikana Thornveld (SVcb 6). It is not a veld type very rich in perennials, grasses and bulbs so the net was cast a little wider to adjacent Veld Types as well.
Plants were selected using the following criteria:
Diversity. It was impotent to allow for diversity in order to mimic the immediate environment but also to accommodate the other variables that were considered. The plants varied from grasses, perennials, bulbs to small shrubs.
Height. Variation was important and the height varied from a few centimeters to approximately 1m.
Root depth. The soil depth of 20cm limited the selection of plants to ones with root balls rather than taproots. Where geophytes were selected, it was ensured that the underground storage organ remains relatively small.
Flowering time. The aim was to have something in flower every month of the year.
Ecological importance. Plants were selected to attract the widest possible range of fauna ranging from birds to butterflies and bees.
Planting was undertaken early in the season to give the plants enough time to establish.
I think the pictures say it all. I also include a list of plants that was initially established but it is by no means static and necessarily valid for the whole of the Highveld.
What is even more impressive about this case study is that the sustainability were applied to fields as well, such as solar power and water management.
Please remember that this case study is applicable on a residential scale and not a business scale
Succulents and Bulbs